This invention relates to wholly solid electric cells and gas sensors and, more particularly, to electric cells and gas sensors using an alkali ion conductive glass-ceramics.
Recent development in electronics has brought about high-performance electronic devices of a compact and light-weight design and, as a power source of such electronic devices, development of an electric cell of a high energy density and a long life is strongly desired for.
Lithium has the highest oxidation-reduction potential of Li/Li.sup.+ of all metal elements and has the smallest mass per 1 mol and, therefore, lithium cell can provide a higher energy density than other types of cells. Moreover, if a lithium ion conductive solid electrolyte is used, a cell of a thin film can be formed and increase in energy density per unit volume can thereby be realized.
A lithium ion cell which has been realized to date uses an organic electrolyte solution as its electrolyte and this makes it difficult to achieve a cell of a compact design such as a thin film design. This lithium ion cell has additional disadvantages that it has likelihood of spontaneous combustion. If this lithium ion cell is replaced by a cell employing an inorganic solid electrolyte, a wholly solid cell of a high reliability will be realized. For this reason, studies and development of a solid electrolyte having a high conductivity have been vigorously made for realizing a wholly solid lithium cell.
Moreover, carbon dioxide gas produced by combustion of fossil fuel is a main cause of a hothouse effect which has recently become a serious problem and it has become necessary to incessantly watch the concentration of carbon dioxide gas. Therefore, establishment of a system for detecting carbon dioxide gas is a matter of increasing importance for the maintenance of a comfortable life in the future human society.
Carbon dioxide gas detection system which are currently in use are generally of a type utilizing absorption of infrared ray. These systems however are large and costly and besides are very susceptible to contamination. For these reasons, studies have recently been actively made to develop a compact carbon dioxide gas sensor using a solid electrolyte. Particularly, many reports have been made about studies using a lithium ion solid electrolyte.
For realizing such gas sensor using solid electrolyte, development of a solid electrolyte which is highly conductive, chemically stable and sufficiently heat proof is indispensable.
Among known electrolytes, Li.sub.3 N single crystal (Applied Physics letter, 30(1977) 621-22) and LiI--Li.sub.2 S--P.sub.2 S.sub.5, LiI--Li.sub.2 S--SiS.sub.4 and LiI--Li.sub.2 S--B.sub.2 S.sub.3 glasses (Mat. Res. Bull., 18(1983) 189) have high conductivity of 10.sup.-3 S/cm or over. These materials, however, have the disadvantages that preparation and handling of these materials are difficult and these materials are not sufficiently heat proof. Particularly, these materials have the fatal disadvantage that decomposition voltage of these materials is so low that, when they are used for an electrolyte of a solid cell, a sufficiently high terminal voltage cannot be obtained.
An oxide lithium solid electrolyte does not have the above described disadvantages and has a decomposition voltage which is higher than 3V and, therefore, it has possibility of usage as a wholly solid lithium cell if it exhibits a high conductivity at room temperature. It is known in the art that conductivity in an oxide glass can be increased by increasing lithium ion concentration. However, there is limitation in increasing the lithium ion concentration even if rapid quenching is employed for glass formation and conductivity of this glass at room temperature is below 10.sup.-6 S/cm at the highest.
Japanese Patent Application Laid-open Publication No. Hei-8-2239218 discloses a gas sensor using a thin film of lithium ion conductive glass. The conductivity of this lithium ion conductive glass thin film is between 1.7.times.10.sup.-7 and 6.1.times.10.sup.-7. This is not a sufficiently high value and a solid electrolyte having a higher conductivity is desired for.
An oxide ceramic having the highest conductivity at room temperature is Li.sub.1+X Al.sub.X Ti.sub.2-X (PO.sub.4).sub.3. When X is 0.3, the conductivity thereof is 7.times.10.sup.-4 S/cm at room temperature (J. Electrochem. Soc. 137(1990) 1023). Oxide ceramics are superior in conductivity to glasses but have the disadvantages that they require a troublesome process for manufacturing and that they are difficult to form, particularly to a thin film.
In short, the prior art lithium ion solid electrolyte have the problem that they are either low in conductivity, hard to handle, hard to form to a compact design such as a thin film.
It is, therefore, an object of the invention to provide an electric cell and a gas sensor of a high performance which have solved these problems.